The present invention relates to a compound sleeve roll suitable for hot and cold rolling and a method for producing it, and more particularly to a crack-resistant compound sleeve roll having a chamfered portion from an outer surface of a shell portion to an end surface of a core portion, and a method for producing it.
The rolls are required to have roll surfaces suffering from little wear, little surface roughening, little sticking with materials being rolled, less cracks and fractures, etc. For this purpose, cast compound rolls having hard outer surfaces and forged steel rolls having roll body portions hardened by a heat treatment, etc. are conventionally used depending on applications.
As further improved wear resistance is required for rolls, compound rolls having shell portions produced from sintered alloys have recently been provided. For instance, Japanese Patent Laid-Open No. 62-7802 discloses a compound roll constituted by a shell portion and a roll core, the shell portion being made from powder of high-speed steels such as SKH52, SKH 10, SKH57, SKD11, etc., high-Mo cast iron, high-Cr cast iron, high-alloy grain cast iron, Ni-Cr base alloy, etc., and diffusion-bonded to the roll core by a HIP treatment.
These rolls produced by sintering alloy powders have been finding wide applications, in place of conventional cast iron rolls, from finish stands to intermediate stands for hot-rolling wires, rods, plates, etc. The rolls produced by sintering alloy powders are superior to the cast iron rolls with respect to wear resistance and resistance to surface roughening, but they are still insufficient in crack resistance.
The above conventional east iron rolls may be reused by grinding to remove heat cracks generated on a shell surface during rolling operations. However, the sintered alloy rolls would be broken if they continue to be used with cracks remaining in the shell portions, because the cracks easily propagate through the rolls.
Japanese Patent Laid-Open No. 2-80109 discloses a compound roll produced by sintering high-alloy powders by a HIP method, in which a transformation stress generated at the time of a heat treatment is relaxed by a special design of the roll. Specifically, this compound roll has a core portion 21 around which a roll body portion 22 is formed, the roll body portion 22 having on both sides annular projections 24, and a hardened layer 23 made of a high-alloy metal showing excellent rolling characteristics being integrally bonded between the annular projections 24. Each annular projection 24 has an annular groove 25 near the axial end 27 of the hardened layer 23 to form a buffer wall portion 26 which acts to relax a transformation stress generated at the time of a heat treatment.
Although a sintered shell portion formed from high-alloy powder to meet the requirement of a high wear resistance is poorer in crack resistance than a core portion having excellent toughness, almost all stresses such as residual stress, rolling stress, heat stress, etc. are borne by the shell portion. Accordingly, cracking is highly likely to take place near the axial end of the roll. For this reason, rolling is usually conducted without permitting an article being rolled to pass through the rolls in a range of about 50 mm or less from each side end of the rolls. This inevitably leads to poor productivity and increased roll cost.
In the compound roll having such a roll shape as to relax a transformation stress on both sides which is disclosed in Japanese Patent Laid-Open No. 2-80109, a width of a hardened layer usable for rolling an article is restricted. Usually, edge portions of the roll on both sides are chamfered to a degree of about C10 (10 mm in axial direction and 10 mm in radial direction). However, these chamfers are made to prevent the roll edges from being broken by impinging other articles in the course of handling, but they do not contribute to relax the stress.